Metal reduction cells, such as those used for producing aluminum, typically utilize carbonaceous cathodes. The cathode can be in the form of a layer formed on the inside of the reduction cell, for example, as an array of a cathode blocks joined by ramming paste. However, over time, electrolyte in the cell and the molten metal tend to attack the carbon-based cathode, causing it to erode. The erosion is further enhanced by movements in the cell due to magneto-hydrodynamic effects. Similar erosion also occurs to the ramming pastes used to seal cracks and joints in the cell.
It has been known for a number of years that cathodes can be made from a composite of a carbon-containing component and a metal boride, such as titanium diboride (TiB2). The TiB2 helps to protect the cathode against erosion and oxidation and makes the cathode wettable to aluminum. The wettability is an important characteristic particularly in drained cathode cells.
Attempts have been made to apply refractory coatings made of metal borides, such as titanium boride (TiB2), to a cathode to protect it from erosion. An example of such coating is described in WO 01/61077, in which the coating was made from a refractory slurry of titanium boride dispersed in an aluminum oxalate complex. However, differences in thermal expansion between the coating and the cathode often cause the coatings to crack or dislodge from the cathode.
Another solution to cathode erosion is described in WO 00/36187 where composite cathodes blocks are formed, in which metal boride layers are bonded to a carbonaceous substrate to form a multi-layer cathode block. The carbonaceous substrate is given a roughened surface so that the metal boride layer may better bond to the carbonaceous substrate.
Since metal borides used in making cathode blocks are very expensive, another method of manufacturing the blocks is to mix metal boride precursors of, for example, metal oxides and boron oxides, with the carbonaceous substrate to produce a composite material that forms metal boride in situ when exposed to molten aluminum in the cell, or when it is exposed to the heat of the cell at start-up and during operation. An example of such a process is described in WO 00/29644.
Although use of cathode blocks containing metal borides in reduction cells reduces the extent of cathode erosions, lab and factory experiments show that metal boride particles gradually leach out of the cathodes and enter a film of liquid aluminum present on the surface of the cathode. In industrial use, this leads to the formation of a metal boride-aluminum layer, having a thickness of approximately 3 mm, on the cathode. In the case where titanium boride is used in the cathode block the layer is a TiB2—Al(l) layer. The removal of TiB2 particles leads to a contamination of the metal product and to a progressive erosion of the cathode blocks, since the more metal boride that leaches out of the cathode, the more quickly the cathode erodes.
It is therefore desirable to find an inexpensive and simple way of preventing leaching of metal borides from carbonaceous composite cathode blocks, refractory coatings and ramming pastes.
It is also desirable to make erosion-resistant, aluminum wettable, cathode blocks, refractory coatings and ramming pastes which do no leach out metal borides during use.